Projector light sources 101: What are the differences?
Conventional digital projectors use ultra-high pressure (UHP) Mercury lamps as light sources, except for cinema projectors which apply Xenon lamps. We will not talk about cinema projectors but only about consumer devices.
UHP lamp-based projectors suffer from containing hazardous substances (Mercury), comparatively low lifetime and thus the need to replace the lamp from time to time. They require cooling even after the lamp was switched off which determines the minimum shut-down time. In addition, the fans for cooling are a source of noise and subject to wear. All these reasons made the projector manufacturers look for other technologies to generate the light required.
One common technology feature of more efficient light sources are lasers. But behind this buzz word there are several different technologies.
Pure laser projectors use lasers for each of the primary colors red, green, and blue. As lasers are currently the most efficient light sources, laser diodes are preferred for small battery driven projectors to increase the on-time per battery charge. But single laser light sources show an unwanted phenomenon: speckle. Speckles result from interference effects of the laser light beam with itself when reflected at any diffusively reflecting surface – including projection screens. It can be avoided only by using several laser diodes per primary color, which makes the projectors more expensive but also brighter – and larger. In cinema this is applied to a large extent, but there are few speckle-reduced consumer laser projectors.
Instead there are many projectors called “laser” that use just one of these light sources to generate blue light. The red and green light required is produced otherwise. On one hand there are so-called laser phosphor projectors which use a part of the blue laser light to excite a substance called phosphor or luminophore to emit yellow light which can be split into green and red. In principle this substance is the same as that in fluorescent lamps. The phosphor – not to be mixed up with the chemical element Phosphorus! – has to be cooled to withstand the high power density of the blue laser light. In general, this is done by rotating a wheel which carries the phosphor – a mechanical part which is prone to wear. But other cooling methods and/or phosphors that withstand higher temperatures are under development. In any case active cooling will reduce the efficiency of the system.
Laser phosphor projectors are a kind of hybrid solution for generating light in red, green, and blue. On the other hand, there is another hybrid solution using a blue laser and a red LED. Again, the green light is produced by conversion of blue laser light using a phosphor wheel. Unless the manufacturer uncovers the light generation principle the user cannot discriminate the different ways because all of them are sold as “laser” projectors. The expert of course can tell by measuring the light spectra.
Still another way of producing red, green, and blue light is using exclusively LEDs. These devices are not quite as efficient as lasers (without cooling) but still much more than Mercury lamps. With LEDs there is no speckle problem at all because their wavelength range is much broader than that of lasers. In order to make it even broader the green LED often is actually a blue one combined with a green phosphor. Here the phosphor does not have to be cooled as the light of an LED is much less concentrated than that of a laser. The only limitation of LEDs currently is the achievable brightness.
Lasers and LEDs both have a much longer lifetime than UHP or Xenon lamps. Start-up is possible almost instantaneously, and shut-down is also much faster than with conventional light sources, especially when no active cooling is required.
Let us come to wavelength multiplex 3D (WMT). This is known to work well with conventional light sources though a lot of light is lost in the filters required. When the emission bands of the primary colors red, green, and blue are narrower but not too narrow to be split into two bands each, the efficiency of a WMT 3D system will increase. This is the case for LED light sources. With laser projectors we have got a problem: Laser lines are so narrow, they cannot be divided into two that can be separated by any kind of filters. The solution for pure laser projectors is simple but costly: double the number of wavelengths used in order to get two blues, two greens, and two reds, one of each for the left image and one for the right one. This technique is called 6P for six primaries and is extremely light efficient. In addition, INFITEC offers lightweight glasses with unbreakable film lenses for these projection systems.
But what about laser phosphor projectors? The original design cannot be used with wavelength multiplex 3D, because a single blue laser line cannot be split into two which could be separated by glasses. But in the meantime, there are first projectors with two different blue laser wavelengths reaching the market. They are specially designed to fit with the wavelength multiplex 3D technology and passive glasses specially designed for these projectors are available from INFITEC.